FORMATION EVALUATION

PETE 321

Summer 2010

David Schechter

DENSITY AND NEUTRON LOGS

Fluorescent Intervals in 1U Sand

• Sharp transition between oil saturated pay and non-pay observed by fluorescence in core samples

GEOLOGICAL AND PETROPHYSICAL DATA USED TO DEFINE FLOW UNITS

FlowUnits

Gamma RayLog

PetrophysicalData

PoreTypesLithofaciesCore

1

2

3

4

5

CorePlugs

CapillaryPressureφ vs k

Modified from Ebanks et al., 1992

POROSITY TOOLS

• Sonic• Density• Neutron

• Introduction to Nuclear logging• Review of basic tools

– Density Tools– Neutron Tools

• Applications/Limitations• Log examples

OVERVIEW

NATURAL RADIOACTIVITY

Spontaneous emission of α and β particles and γradiation from the nucleus of a atom

• Alpha particle = 2 neutrons + 2 protons(same as helium nucleus)

• Beta particle = high velocity electrons

• Gamma rays = electromagnetic radiation(no mass or charge; energy in MeV)

FAMILY OF NUCLEAR TOOLS

Density toolsGR tools Neutron tool

GasOil

GR Neutrons

Natural GR Emitted GREmittedNeutrons

ShalinessCorrelation

PorosityDensity

LithologyPorosity

Particle

Source

Application

DENSITY LOGS

From Halliburton (EL – 1007)

• Uses– Density– Porosity– Lithology

• Curves– Bulk density

(ρb and Δρ)– Pe

DENSITY & POROSITY MEASUREMENTS

COMPENSATED DENSITY LOG

ρcφDρe ρb

Why has Sonic been displaced as phi tool

• Porosity can be determined without precise

knowledge of rock matrix

• No need for compaction correction

• Overlay of density-neutron excellent indicator of

gas

• Transitions from one rock type to another

detected

• Shale effects more evident

DENSITY PRINCIPLE• Detect GR’s from the source which have been

scattered back by the formation

FormationHydraulic

sonde

Caliperarm

Skid

Detectors

Gamma rayemitting source

Gamma rays

Low DensityWindows

CesiumSource

Stabilizer

TungstenBore Liner

Far Detector

Near Detector

TungstenShield

PRINCIPLE

• Gamma rays emitted from radioactive source

• Gamma rays collide with electrons in formation,

losing energy

• Detectors measure intensity of backscattered

gamma rays

– High energy GR relate to - Density

– Low energy GR relate to - Lithology

GAMMA-RAY GENERATION

Chemical source- Focused- Typical properties

Cesium -137 material1.7 Curie strength33 year half-life662 keV gamma-ray energy

GAMMA RAY INTERACTIONSWITH MATTER

• Pair Production

• Compton Scattering

• Photoelectric Adsorption

PAIR PRODUCTION

Not of significance to density tool operation since the source strength is 0.662 MeV and it requires 1.02 MeV for pair production ( 0.51 MeV is the energy equivalent

of the mass of one electron )

GAMMA RAY SCATTERINGCompton EffectMedium to High Energy GR’s

- scattered by electrons information

- each interaction loses energy- more electrons => more

scattering- Related to elect. & rock den.

e-

Eo=hνo X ray

Eo=hνo

e-

E’=hν’

Photoelectric EffectLow Energy GR’s

- absorbed by atoms- more electrons => more absorption- Indicates the atomic number - lithology

Nucleus

COMPTON SCATTERING

Measuring the number of gamma rays in the Compton Scattering energy range gives us the bulk density

PHOTOELECTRIC ABSORPTION

• The number of gamma rays capable of photoelectric absorption indicates the Atomic Number and hence the Pe

• This information indicates the lithology

WHAT THE DETECTOR “SEES”

Density Region(Compton scattering)

Lithology Region(Photoelectric effect)

Source

EFFECT OF FORMATION DENSITY ON DETECTOR COUNT RATES

At. No. same; different density

EFFECT OF FORMATION ATOMIC NUMBER ON DETECTOR COUNT RATES

Density same; At. No. different

DENSITY LOGS

• Bulk density, ρb, is dependent upon:• Lithology• Formation porosity• Density and saturation of fluids in

pores

GRAPI0 200

CALIXIN6 16

CALIYIN6 16

RHOBG/C32 3

DRHOG/C3-0.25 0.25

4100

4200

DENSITY LOG

Caliper

Density correction

Gamma ray Density

From NExT, 1999

BULK DENSITY INTERPRETATION - 1

φρρφρ +−= )1( ma flb

• The bulk density (in g/cc) is the weighted sum of the matrix and fluid densities

• Typical values• Matrix density 2.65 SS; 2.71 LS; 2.87 Dol• Fluid density

• 0.9 – 1.0 OBM and fresh WBM• 1.1 - 1.2 salty WBM

• Density porosity curve is derived from above equation

BULK DENSITY INTERPRETATION - 2

ρρρρ

φ−

−=

ma fl

ma bD

• Charts– POR-5 (S)– POR-10 (H)

• Rearranging the equation gives POROSITY

2.31 g/cc

25%

BULK DENSITY INTERPRETATION - 3• The density correction (Δρ) curve is

“measurement quality”• Poor pad contact gives Δρ > 0.05• Often correlates with caliper

Δρ

-0.25 0 +0.25

CAL

If correction > 0.20 g/ccBulk density curve is invalid

PHOTOELECTRIC INTERPRETATION - 1

flma

flflmama

ee

eeeee

PPP

φρρφ

φρρφ

+−

+−=

)1(

)1(

• The Pe value (in barns/electron) is the weighted sum of the matrix and fluid capture cross sections

• Typical Values• Matrix Pe: 1.8 SS; 5.1 LS; 3.1 DOL• Matrix ρePe: 4.8 SS; 13.8 LS; 9 DOL• Fluid ρePe:

• 0.1 - 0.4 OBM and fresh WBM• 0.4 – 1.0 salty WBM

PHOTOELECTRIC INTERPRETATION - 2

• The logging curve is Pe• The product ρePe = U, capture cross-section/cc

flma UUU φφ +−= )1(

• This looks like the density equation• We don’t solve for φ because Ufl << Uma

• See Appendix 4 Charts for values of Pe

TYPICAL FORMATIONS

Sandstone

Limestone

Dolomite

PEFRHOB

0 1032

2.65

2.87

2.715.1

3.1

1.8

Matrix values

SCHLUMBERGER WIRELINE TOOL HISTORY

Three-Detector Lithology Density (TLD)

Powered Gamma Tool (PGT)

Nal(TI)Scintillation DetectorGeiger-Muller DetecterGamma Ray Source

Nal(TI) Scintillation DetectorNal(TI) Scintillation DetectorGamma Ray Source

Nal(TI) Scintillation DetectorNal(TI) Scintillation DetectorGSO(Ce) Scintillation DetectorGamma Ray Source

Litho Density Tool (LGT)

Flex Joint

FlexJoint

FACTORS AFFECTING DENSITY LOG RESPONSE

• Shales and clays• May cause porosity reading to be too

high or too low• Vsh and ρsh can be obtained from log

readings in shale zones• Hydrocarbons

• In oil zones, ρhc = ρo which can be measured from fluid samples

• In gas zones, ρhc = ρg which can be measured or calculated using gas properties

• Gas will cause anomalously low density and, thus, high density porosity ρρ

ρρφ

−

−=

ma fl

ma bD

DENSITY LOG COMMENTSGRAPI0 200

CALIXIN6 16

CALIYIN6 16

RHOBG/C32 3

DRHOG/C3-0.25 0.25

4100

4200

Caliper

Density correction

Gamma ray

Density

From NExT, 1999

If correction > 0.20 g/ccBulk density curve is invalid

• Very reliable tool• Used to determine

– bulk density– Porosity– lithology

• Shallow depth of investigation - 10 to 15 cm

NEUTRON LOGS

NEUTRON LOGSUses of neutron logs• Identify porous zones• Determine porosity• Identify gas in porous zones

Where neutron logs can be used• Any borehole

• Open or cased• Liquid- or air-filled

Depth of investigation• 6-12 inches for CN

NEUTRON MEASUREMENT

• Uses• Lithology

Porosity

• Curve φN

Pe

ρb ΔρφN

NEUTRON TOOL PRINCIPLE

• Detects neutrons from the source which have been scattered back by the formation

• Source AmBe 15-20Cu 5MeV

•The neutron tool employs a dual detector design to compensate for mudcake, lithology, etc.

•Still, corrections are required for the NPHI values

•NOTE : The tool is pressed against the borehole wall to minimize mud effects

LIFE OF A NEUTRON - 1

• Neutrons emitted from source• Neutrons interact with Hydrogen in formation• Neutrons lose energy• Neutrons are absorbed or reflected back to

detectors– High counts = Low porosity– Low counts = High porosity

LIFE OF A NEUTRON - 2

• Source AmBe 15-20Cu 5MeV neutrons

• Collisions cause neutrons to lose energy

• Energy loss due mainly to hydrogen

• Therefore tool measures amount of hydrogen in formation, ie., water, oil

Thermal Neutrons

• The neutron tool responds primarily to the presence of hydrogen

• The more hydrogen, more neutrons slowed to the thermal level and captured by the formation

• Other minerals also have a small effect on the neutron tool, which requires compensation

NEUTRON TOOL DESIGN• Both detectors placed in

long spacing zone

• Count rates at the detectors are inversely proportional to formation porosity and distance from source

• Near/Far Ratio proportional to porosity

• Ratio compensates for mudcake

ENVIRONMENTAL EFFECTS ON NPHI

Lithology Effects

• The tool responds mostly to the presence of hydrogen, but different minerals will slow neutrons to different degrees

• Therefore, the Neutron tool reads different values for NPHI in different formations of the same porosity

• This must be taken into account for the NPHI curve

NEUTRON LOG INTERPRETATION - 1• Ratio converted to

apparent porosity, φN• Many environmental

effects• Assumes

• Matrix• Usually LS• Sometimes SS

• Water-filled• Charts POR-12--16

• Chart varies with tool

Question:

On a limestone scale, the NPHI is 20%. However the formation is a sandstone. What is the true porosity?

Answer : 25%.

GAS EFFECTS ON NPHI• Gas has a lower hydrogen concentration

than oil or water due to it’s lower density

• A neutron tool interprets gas to be water

occupying a smaller volume; a smaller

volume means a smaller porosity

• Hence in gas zones, the neutron tool reads

anomalously low porosity

NEUTRON LOG INTERPRETATION - 2• Reads deeper than density

• More affected by virgin zone fluid• Gas effect

• Gas lowers H concentration, lowers apparent porosity

Gas effect

-3+9+15+21φ N

Notice the NPHI reading is less in a gas zone than in an oil zone in the same lithology

In a gas zone, NPHI reads too low and DPHI reads too high

The 2 curves track closely in oil-saturated zone

GASEFFECT

SHALE EFFECTS ON NPHI

• Shaliness affects neutron porosity• Shale has bound water in lattice structure• This water is immobile and does not represent

EFFECTIVE porosity• However, the neutron tool responds to the presence

of hydrogen in the bound water of shales, and the neutron tool reads anomalously high NPHI

NEUTRON LOG INTERPRETATION - 3• Shale effect

• Responds to bound water

Shale effect

Por. = 39%

0

Each PHIN division = 3%

NEUTRON SHALE EFFECT

PEF• Sand – Clean ~ 1.8• Sand – Dirty ~ 2.7• Average Shale ~3.4

NPHI SCALES• NPHI is usually plotted on a limestone scale

or a sandstone scale • If on a limestone scale, in a 100% water

bearing limestone, the neutron reads the correct porosity; in a 100% sandstone, the porosity is wrong and must be corrected for lithology

• If on a sandstone scale, in a 100% water bearing sandstone, the neutron tool is reads the correct porosity; in a limestone, readings must be corrected for the lithology

High NPHI across shales

On a limestone scale, it reads actual porosity in limes

In sands, it reads a different porosity from the actual

SHALEEFFECT

NEUTRON LOGSUses of neutron logs• Identify porous zones• Determine porosity• Identify gas in porous zones

Where neutron logs can be used• Any borehole

• Open or cased• Liquid- or air-filled

Depth of investigation• 6-12 inches for CN

SUMMARY• Nuclear porosity tools

• Source• Detectors

• Density• Bulk density• Photoelectric effect• Quality curve• Flushed zone measurements

• Neutron• Apparent porosity• May respond to virgin zone